Strobl G, Cho T Y
Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, 79104 Freiburg, Germany.
Eur Phys J E Soft Matter. 2007 May;23(1):55-65. doi: 10.1140/epje/i2007-10177-4. Epub 2007 May 24.
Temperature-dependent measurements of spherulite growth rates carried out for i-polystyrene, poly(epsilon -caprolactone) and linear polyethylene show that the controlling activation barrier diverges at a temperature which is 14K, 22K and 12K, respectively, below the equilibrium melting points. We discuss the existence of such a "zero growth temperature" T(zg) in the framework of a recently introduced thermodynamic multiphase scheme and identify T(zg) with the temperature of a (hidden) transition between the melt and a mesomorphic phase which mediates the crystal growth. The rate-determining step in our model of crystal growth is the attachment of chain sequences from the melt onto the lateral face of a mesomorphic layer at the growth front. The necessary straightening of the sequence prior to an attachment is the cause of the activation barrier. A theory based on this view describes correctly the observations. With a knowledge of T(zg) it is possible to fully establish the nanophase diagram describing the stability ranges of crystalline and mesomorphic layers in a melt. An evaluation of data from small-angle X-ray scattering, calorimetry and optical growth rate measurements yields heats of transition and surface free energies of crystals and mesophase layers, as well as the activation barrier per monomer associated with the chain stretching. According to the theory, the temperature dependence of the crystallization rate is determined by both the activation energy per monomer and the surface free energy of the preceding mesomorphic layer. Data indicate that the easiness of crystallization in polyethylene is first of all due to a particularly low surface free energy of the mesomorphic layer.
对等规聚苯乙烯、聚（ε-己内酯）和线性聚乙烯进行的球晶生长速率的温度依赖性测量表明，控制活化能垒在分别低于平衡熔点14K、22K和12K的温度下发散。我们在最近引入的热力学多相体系框架内讨论这种“零生长温度”T(zg)的存在，并将T(zg)确定为熔体与介导晶体生长的介晶相之间（隐藏）转变的温度。我们晶体生长模型中的速率决定步骤是熔体中的链序列附着到生长前沿介晶层的侧面。附着前序列的必要伸直是活化能垒的原因。基于这种观点的理论正确地描述了这些观察结果。了解T(zg)后，就有可能全面建立描述熔体中晶体和介晶层稳定范围的纳米相图。对小角X射线散射、量热法和光学生长速率测量数据的评估得出了转变热、晶体和中间相层的表面自由能，以及与链拉伸相关的每个单体的活化能垒。根据该理论，结晶速率的温度依赖性由每个单体的活化能和先前介晶层的表面自由能共同决定。数据表明，聚乙烯结晶的容易程度首先归因于介晶层特别低的表面自由能。
